2009-12-21

Opportunity Sol 324 (Spirit Sol 344)

Despite my misgivings about it, the drive appears to have gone splendidly. The heat shield looks significantly closer, though range data indicates it's still about 130m away. There are a lot of jokes about this being like a dream: the closer we get to it, the farther it recedes. Some of us -- those who are hoping this isn't our heat shield but some alien artifact[1] -- prefer the explanation that it's moving away from us to stay at the edge of our sensor range. Next thing you know, it will be modulating its shield harmonics.

However, we're not sure exactly how far away it is. Estimates range from 100m to 150m, my personal favorite being the 132m estimate the PANCAM data suggests. Since we're trying to get 40m away as of this next drive, and we also want to keep at least 20m away in order to avoid a possible debris field, this uncertainty is a problem. I decide to drive 90m in its direction, but aimed slightly to the right. That way, if the range to the heat shield is on the low side, we'll still be safe; if I'm right, we'll be smack where we want to be.

It's a gamble, but it's one I feel really good about. Opportunity is eating distance like it was born to (which, come to think of it, is true), and the PANCAMs have been pretty reliable in these circumstances before.

What I don't know, until after I've already planned the drive, is that 90m is a record-setting drive. Not in terms of one-sol distance, but it's the longest blind drive ever -- all previous drives longer than that have used autonav for anything past about 70m. And I thought the days (sols) of record-setting drives were over!

The lengthy blind drive makes some people nervous. To my own surprise, I manage to talk Jeff Biesiadecki (RP-2 today) into it, but Mark Maimone is a tougher sell. Jeff has to leave to support the ongoing depotatoization efforts on the other side of the planet, but Mark and I stay upstairs and argue -- or, as I try to think of it, negotiate.

I look at the terrain and see flat, flat, flat all the way to the horizon. Mark's more conservative: he agrees that what we can see looks flat, but he points out that our imaging resolution diminishes rapidly after 60m or so, and it's theoretically possible there are hazard-sized dips along our path that would be invisible from our starting location.

Unlikely? Sure, but I have to admit it's possible, for sufficiently small values of "possible." We've already configured the rover to stop if it finds itself tilting more than 12 degrees, or if the suspension articulates significantly, which should catch problems like this. But Mark doesn't feel that the limit is set tight enough. So we look back at the actuals from the past few plain drives and choose newer, even tighter limits that we tell the rover to install after it's gone 65m. (Six degrees of tilt or a 6.5-degree bogie angle, and the rover will stop cold. With those limits, it won't take much more than a pebble to halt us.) So it'll be "normally conservative" for the first two-thirds of the drive and ultra-conservative after that. Neither of us is really happy with the result, which is always the sign of a good compromise, so I let it go.

When Jeff returns from the far side of the world, I bring him up to speed on the changes. He notices an extra command I added when making these modifications -- harmless, but unnecessary -- and since it's so late in the process, we just leave it in. When I shake my head in mock-disgust at my mistake, he tries to console me.

"Well, it's harmless," he points out.

"It'll take one second to execute, though." (All commands take at least that long.)

"I did the math once," he says. "I think I figured the rover cost about ten thousand dollars per second during the nominal mission. Of course, it's cheaper than that now."

"Hey, that gives me an idea! We could have each command sponsored by a corporation. Put it in the comments field: 'This command brought to you by Intel.' We'd make a mint!"

If only.

(For the record: The rovers cost about $45 per second during the nominal mission -- that's $4 million per sol, divided by the number of seconds in a sol. The extended mission has reduced the cost by about a factor of three so far, so they're maybe $15 per second.[2] So I guess I owe the project $15 for my mistake. Let 'em take it out of my paycheck.)

Jeff also shows me a video of the rover driving in the testbed with limits similar to the ones Mark and I agreed on. This is disheartening: it seems to stop the instant one wheel begins to dip into a hole. Indeed, the limits aren't far off what the rover can achieve, even on flat terrain, by just steering the wheels to the turn-in-place configuration. Partly, I'm worried that this will ruin my chance to set another record, but that's not a big deal -- I've had my share of fun setting interplanetary records, and then some.

I'm more concerned about the possibility of needlessly delaying science experiments, but I decide the right thing to do is to let it go. Since sol 1, my policy has been to honor the most conservative (yet still reasonable) engineering analysis. Today that happened to be Mark's. ("I can't remember the last time I wasn't the most conservative RP," Jeff comments.) I can't help chafing at this particular result, since I disagree with the analysis, but I firmly believe the policy's the right one. So if the drive gets cut short, we'll just have to live with it. We have another sol before we need to be in our final weekend position anyway, and if the drive is cut off 25m short, we should still be able to make it.

"We'll just have to cross our fingers, I guess," I say to Jeff.

"Yeah," he says, "but I'd rather be crossing my fingers this way than the other way, you know what I mean?"

Sad to say, I do. Ah, we'll be fine. Besides, the numbers we picked would have let us do the last couple of sols of driving, so since part of my own argument is that the road ahead is like the one behind, we should have nothing to worry about.

Not that that'll stop me.

Just before the CAM, we get a new image: a beautiful color PANCAM showing the heat shield, apparently broken and bent from the impact. I suddenly want to see the damn thing up close. I have to see it!

Jeff laughs. "Hey, if the command dictionary allows it, it must be safe, right?"

Courtesy NASA/JPL/Cornell. One of several approximate-true-color PANCAM images we took as we approached Opportunity's heat shield, and probably the one I was looking at then. (Follow the link for the full-sized image.)